Cellulosic materials, including wood pulp for paper making and cotton fibers in the manufacture of textiles, require bleaching. One method of bleaching wood pulp employs an alkaline system using hydrogen peroxide. The factors affecting such processes are described in Pulp & Paper, June 1980, pp. 156-161. Alkalinity is one factor, e.g. high pH favors the bleaching process, but also accelerates the decomposition of the peroxide which wastes the bleaching agent. The control of metal ions to prevent their interaction with the peroxide is another factor. This is accomplished by the addition of chelating agents. Temperature, pulp density and type of wood are other factors which affect the brightness in the process of bleaching thermal mechanical pulp with hydrogen peroxide.
In the process of making wood pulp, metal ions can enter the system from several sources including the wood itself, the water and the machinery used to masticate the wood chips and pulp. While some of the metal ion content is lost in the deckering or dewatering step, it is sometimes an advantage to add a chelating agent. Of all the commercially available chelating agents, the one reported to be the most effective is the sodium salt of diethylenetriaminepentaacetic acid (DTPA). This is found in an article titled "The Effect of DTPA on Reducing Peroxide Decomposition", D. R. Bambrick, TAPPI Journal, June 1985, pp. 96-100. Silicates are commonly used as peroxide stabilizers in the bleach liquor. The use of silicates in such systems, however, results in insoluble silicates being deposited upon the machinery employed as well as the pulp fibers. When deposited on the pulp fibers the result is a harsher feel of the paper while the fouling of equipment can cause down-time and a shorter life for the equipment. Because of this, silicate-free systems have been suggested as an alternative.
These silicate-free systems have been found to work well in the single stage hydrogen peroxide bleaching of Kraft pulps where the choice of stabilizer possibly influences the bleaching mechanism by changing the reaction pathway of hydrogen peroxide. In such systems, the addition of poly(.alpha.-hydroxyacrylate) as a stabilizer also has been shown to improve pulp brightness. British patent 1,425,307 discloses a method for preparing this stabilizer. The use of this stabilizer is discussed in a paper "Hydrogen Peroxide Bleaching of Kraft Pulp and the Role of Stabilization of Hydrogen Peroxide", by G. Papageorges, et. al., given at the ESPRA Meeting in Maastricht, Netherlands, May, 1979.
In U.S. Pat. No. 3,860,391 the bleaching of cellulose fibers and mixtures thereof with synthetic fibers is accomplished by employing peroxide in a silicate-free system in the presence of an aliphatic hydroxy compound, an aminoalkylenephosphonic acid compound and, alternatively, with the addition of a polyaminocarboxylic acid. Representative of the above are erythritol or pentaerythritol, ethylenediaminetetra(methylenephosphonic acid) or 1-hydroxpropane-1,1,3-triphosphonic acid and ethylenediaminetetraacetic acid or nitrilotriacetic acid, respectively.
U.S. Pat. No. 4,238,282 describes a pulp bleaching system employing chlorine (not peroxide) which uses various chelating agents, including acrylic acid polymers of &lt;2000 mol. wt., alkylene polyaminocarboxylic acids and aminophosphonic acids and their salts.
Another patent (U.S. Pat. No. 4,239,643) and its divisional (U.S. Pat. No. 4,294,575) employ phosphonic acids, such as indicated above, in a peroxide bleaching system. The above two patents include as the stabilizer for the peroxide a combination of an alkali metal polyphosphate and an alkali metal diethylenetriaminepenta(methylenephosphonic acid). The weight ratio of polyphosphate to phosphonic acid used varies from 10:1 to 1:5.
While, as noted above, various combinations of chelating agents are useful in stabilizing peroxide bleaching systems, the presence of metal ions, e.g. iron, manganese and copper, provides a catalytic effect with respect to the decomposition of the peroxide and also tends to reduce the brightness of finished mechanical pulps. While the chelants might be expected to take care of minor amounts of the metal ions, the presence of significant amounts of magnesium and/or calcium ions which may be present in the wood pulp or water or both tends to overwhelm the ability of the chelants to complex the iron, manganese and copper ions.
In a tower bleaching process chelating agents (chelants) such as aminocarboxylic acids, e.g. DTPA, are added prior to the deckering (dewatering) step. A large percentage of the metal ions, therefore, are removed as metal chelates in the deckering process. Additional chelants can also be added in the bleaching step as shown in U.S. Pat. No. 4,732,650 wherein the pulp is treated with a polyaminocarboxylic acid and, after a dewatering step, bleached with a stabilized alkaline peroxide aqueous solution (U.S. Pat. No. 4,614,646) containing a combination of an aminophosphonic acid chelant together with a polycarboxylic acid, a polycarboxylic amide or a sulfonic acid derivative of a polyamide.
In contrast to the tower process defined above, however, the pulp is not dewatered prior to the bleaching step in a typical refiner bleaching process Thus, chelants can be added prior to and/or with the bleach solution to control the effect of transition metals in the pulp slurry of the refiner process.